1. Field of the Invention
The present invention relates to a ranging apparatus and a ranging method, and more particularly to a ranging apparatus and a ranging method for detecting the phase delay of reflected light from an object that is irradiated with modulated light at each of the pixels of an image capturing device, for thereby detecting a three-dimensional structure of the object.
2. Description of the Related Art
One known process for measuring the distance up to an object is an optical TOF (Time Of Flight) ranging process.
As shown in FIG. 18 of the accompanying drawings, a ranging apparatus based on the optical TOF ranging process comprises a light source 200 in the form of an LED array, for example, for emitting intensity-modulated light (modulated light), an image capturing device 204 for detecting reflected light from an object 202 irradiated with the modulated light from the light source 200, and an optical system 206 for focusing the reflected light onto the image capturing device 204.
If the modulated light emitted from the light source 200 and applied to the object 202 is intensity-modulated, for example, at a high frequency of 20 MHz, then the modulated light has a wavelength of 15 m. When the modulated light travels back and forth over a distance of 7.5 m, the modulated light as it is detected by the image capturing device 204 has undergone a phase delay of one cycle length.
The phase delay that the reflected light undergoes with respect to the modulated light will be described below with reference to FIG. 19 of the accompanying drawings.
As shown in FIG. 19, a reflected light R has a phase delay of φ with respect to a modulated light W. In order to detect the phase delay of φ, the reflected light R is sampled at four equal intervals, for example, in one cyclic period of the modulated light W. If the sampled amplitudes of the reflected light R at respective phases of 0°, 90°, 180°, 270°, for example, of the modulated light W are represented by A0, A1, A2, A3, respectively, then the phase delay of φ is expressed by the following equation:φ=arctan {(A3−A1)/(A0−A2)}
The reflected light from the object 202 is focused onto the light-detecting surface of the image capturing device 204 by the optical system 206. The light-detecting surface of the image capturing device 204 comprises a two-dimensional matrix of pixels (photodiodes). When the phase delay of φ is determined at each of the pixels according to the above equation, a three-dimensional structure of the object 202 can be detected.
A ranging apparatus based on the above principle is disclosed in Japanese Laid-Open Patent Publication No. 2006-105694, for example.
In order to avoid a detection error caused by reflected light (delayed reflected light) which has reached the image capturing device with a delay greater than one cyclic period of the modulated light, the disclosed ranging apparatus does not employ those pixels which have detected amounts of light equal to or lower than a preset threshold value. In other words, the disclosed ranging apparatus is unable to calculate distance values for the pixels which have detected the delayed reflected light, among all the pixels of the image capturing device.